In a variety of MRI applications, motion of the examined object (the patient) can adversely affect image quality. Acquisition of sufficient MR signals for reconstruction of an image takes a finite period of time. Motion of the object to be imaged during that finite acquisition time typically results in motion artifacts in the reconstructed MR image. In conventional MR imaging approaches, the acquisition time can be reduced to a very small extent only, when a given resolution of the MR image is specified. In the case of medical MR imaging, motion artifacts can result for example from cardiac cycling, respiratory cycling, and other physiological processes, as well as from patient motion. In dynamic MR imaging scans, the motion of the examined object during data acquisition leads to different kinds of blurring, misregistration, deformation and ghosting artifacts.
Prospective motion correction techniques such as the so-called navigator technique have been developed to overcome problems with respect to motion by prospectively adjusting the imaging parameters, which define the location and orientation of the volume of interest within the imaging volume. In the navigator technique hereby, a set of MR navigator signals is acquired from a spatially restricted volume (navigator beam) that crosses the diaphragm of the examined patient. For registering the MR navigator signals, so-called 2D RF pulses may be used. These excite the spatially restricted navigator volume, for example of pencil beam shape, which is read out using a gradient echo. Other ways to detect the motion-induced momentary position of the volume of interest is the acquisition of two-dimensional sagittal slices that are positioned at the top of the diaphragm, or the acquisition of three-dimensional low-resolution data sets. The respective navigator volume is interactively placed in such a way that a displacement value indicating the instantaneous position of the diaphragm can be reconstructed from the acquired MR navigator signals and used for motion correction of the volume of interest in real time. The navigator technique is primarily used for minimizing the effects of breathing motion in body and cardiac exams where respiratory motion can severely deteriorate the image quality. Gating and image correction based on the MR navigator signals was introduced to reduce these artifacts.
The afore-described navigator technique can generally be applied in different fields of MR imaging in order to detect a specific change in imaging conditions. A further example is the triggering of an imaging sequence after the bolus arrival of a contrast agent at a specific organ of interest.
Subsequent to the measurement of the MR navigator signals, usually a series of phase-encoded spin echoes is generated by an appropriate imaging sequence of RF pulses and magnetic field gradient pulses. These spin echoes are measured as MR imaging signals for reconstructing an MR image therefrom, for example by 2D Fourier transformation.
As mentioned before, the restricted navigator volume is ideally placed over the interface (localized at the dome of the right hemidiaphragm) between the liver and the lung in order to detect the breathing state of the examined patient. This is because of the high MR signal contrast between the lung and the liver. Particularly in abdominal applications, the problem arises that the volume of interest, from which the MR imaging signals are acquired, partially overlaps with the navigator volume. Usually, the acquisition of the MR imaging signals is interleaved with the acquisition of the MR navigator signals without temporal delay. As a consequence, the nuclear magnetization within the volume of interest remains saturated after measuring the MR imaging signals. The resulting saturation bands in the MR navigator signals lead to a wrong detection of the contrast edge indicating the position of the diaphragm. For this reason, the known navigator methods are difficult to apply for MR imaging of the liver or the kidneys. It can not be avoided that the navigator volume is (at least partly) superimposed upon the respective volumes of interest, with the negative consequence that the image quality is considerably degraded due to the incorrect detection of the respiratory motion state
WO 2008/041060 A1 addresses the problem that the nuclear magnetization within the restricted navigator volume remains saturated after measuring the MR navigator signals. In this case, the remaining saturation has the negative consequence that the navigator volume appears as a saturated region in the reconstructed MR images. It is proposed in the cited document to apply a navigator unlabeling sequence prior to generating the actual imaging or spectroscopic sequence. The effect of the navigator unlabeling sequence is that the nuclear magnetization within the restricted navigator volume is converted back into longitudinal magnetization. In this way, the acquisition of MR imaging signals starts without disturbance by the navigator. However, the problem remains that the nuclear magnetization within the volume, from which the MR imaging signals are acquired, remains saturated when the imaging sequence and the navigator sequence are repeatedly applied in an interleaved fashion. The cited document does not propose a solution for the above-mentioned problems associated with incorrect motion detection due to saturation bands in the MR navigator signals.